Six laboratories, including the Plasma Derivatives Laboratory, DBBP, participated in a collaborative study to estimate the variability of the Hepatitis B Immune Globulin (HBIG) potency assay. Samples of five lots were assayed against the HBIG Reference to determine their relative potencies. Each lot was assayed on six different days by each laboratory; the raw data from these assays were supplied to CBER for calculations of the relative potencies (RP). A statistical evaluation of these data provided an overall estimate for the standard deviation (SD) of the log RP equal to 0.054. Since this pooled SD has a sufficiently large degree of freedom (150), it can be taken as the theoretical variability of the assay method. Using this SD, it is possible to define a lower limit for the geometric mean RP of an HBIG lot that will provide 95% confidence that the true RP is greater than or equal to 1.0. For 2, 3 or 4 independent assays, this lower limit is 1.16, 1.13, or 1.11, respectively. Additionally, this study allows us to define the validity of an assay series from the calculated SD of the log RP values.